1. Field of the Invention
The present invention relates to an improved fuse link and, more particularly, to an improved fuse link which is less expensive to manufacture from both a material and a labor standpoint than prior art fuse links and which exhibits improved operating performance notwithstanding the decrease in expense thereof. The improved fuse link of the present invention may be used with a cutout of the type described and claimed in commonly assigned, co-filed U.S. patent application, Ser. No. 132,924 filed Mar. 24, 1980 in the name of Bruce A. Biller. The cutout of the last named patent application may include an improved fuse tube with which the improved fuse link of the present invention may be used, the fuse tube being described and claimed in commonly assigned, co-filed U.S. patent application, Ser. No. 132,922 filed Mar. 24, 1980 in the name of William E. Schmunk.
2. Discussion of the Prior Art
Fuse cutouts and fuse links therefor are well known. A typical fuse cutout includes a hollow insulative fuse tube having conductive ferrules mounted to the opposite ends thereof. One ferrule (often called the "exhaust" ferrule) is located at an exhaust end of the fuse tube and usually includes a trunnion casting which interfits with a trunnion pocket of a first contact assembly carried by one end of a procelain or similar insulator. The other ferrule is normally held by a second contact assembly carried by the other end of the porcelain insulator so that the fuse tube is normally parallel to, but spaced from, the porcelain insulator. The porcelain insulator is mountable to the cross-arm of a utility pole or a similar structure. The fuse link, described in greater detail below, is located within the fuse tube with its ends respectively electrically continuous with the ferrule. One point of an electrical circuit is connected to the first contact assembly, while another point of the circuit is connected to the second contact assembly. Often, the insulator and the fuse tube are generally oriented perpendicular to the ground so that the one ferrule and the first contact assembly are located below the other ferrule and the second contact assembly.
The fuse tube may include a high burst strength outer portion--for example, a fiber-glass-epoxy composite--lined with or containing an ablative arc-extinguishing material, such as horn fiber or bone fiber.
Normal currents flowing through the electrical circuit flow without affecting the fuse link. Should a fault current or other over-current, to which the fuse link is designed to respond, occur in the circuit, the fuse link operates as described below. Operation of the fuse link permits the other or upper ferrule to disengage itself from the second or upper contact assembly whereupon the fuse tube rotates downwardly due to the coaction of the trunnion casting and the trunnion pocket. If the fuse link operates properly, current in the circuit is interrupted and the rotation of the fuse tube gives a visual indication that the cutout has operated to protect the circuit
Typical fuse links include a first terminal and a second terminal, between which there is normally connected a fusible element made of pure silver, silver-tin, or the like. Also connected between the terminals may be a strain wire for a purpose described below. The second terminal is electrically continuous with, and is usually mechanically connected to, a button assembly, which is engageable by a portion of the upper ferrule on the fuse tube. The first terminal is connected to a flexible, stranded length of cable. Surrounding at least a portion of the second terminal, the fusible element, the strain wire (if used), the first terminal, and some portion of the flexible stranded cable is a sheath. The sheath is typically made of a so-called ablative arc-extinguishing material (such as horn fiber) or is a cellulosic material impregnated with an ablative arc-extinguishing material. Such ablative arc-extinguishing materials are well known and typically comprise compounds or compositions which, when exposed to the heat of a high-voltage arc, ablate to rapidly evolve large quantities of de-ionizing, turbulent and cooling gases. Typically, the sheath is much shorter than the fuse tube and terminates short of the exhaust end of the fuse tube.
The free end of the stranded cable exits the fuse tube from the exhaust end thereof and has tension or pulling force maintained thereon by a spring-loaded flipper on the lower ferrule. The tension or pulling force exerted on the cable by the flipper attempts to pull the cable and the first terminal out of the sheath and out of the fuse tube. The force of the flipper is normally restrained by the strain wire, typical fusible elements not having sufficient mechanical strength to resist this tension or pulling force.
In the operation of typical cutouts containing prior art fuse links, the fault current or other over-current referred to above results first in the melting or vaporization of the fusible element, followed by the melting or vaporization of the strain wire. Following such melting or vaporization, a high-voltage arc is established between the first and second terminals within the sheath and the flipper is now free to pull the cable and the first terminal out of the sheath and ultimately out of the fuse tube. As the arc forms, the arc-extinguishing materials of the sheath begins to ablate and high quantities of deionizing, turbulent and cooling gases are evolved. The movement of the first terminal under the action of the flipper, and the subsequent rapid movement thereof due to the evolved gases acting thereon as on a piston, results in elongation of the arc. The presence of the de-ionizing, turbulent and cooling gas, plus arc elongation, may, depending on the level of the fault current or other over-current, ultimately result in extinction of the arc and interruption of the current at a subsequent current zero. The loss of the tension on the stranded cable originally effected by the flipper permits the trunnion to experience some initial movement relative to the lower contact assembly which permits the upper ferrule to disengage itself from the upper contact assembly. This initiates the downward rotation of the fuse tube and its upper ferrule to a so-called "drop out" or "drop down" position.
As noted immediately above, arc elongation within the sheath and the action of the evolved gases may extinguish the arc. At very high fault current or over-current levels, however, arc elongation and the sheath may not, by themselves, be sufficient to achieve this end. Simply stated, at very high fault current levels, either the sheath may burst (because of the very high pressure of evolved gas) or insufficient gas may be evolved therefrom to quench the high current level arc. For these reasons, the fuse tube is made of, or is lined with, the ablative arc-extinguishing horn fiber or bone fiber, as noted above. In the event the sheath bursts, the arc-extending material of the fuse tube interacts with the arc; gas evolved as a result thereof effects arc extinction. If the sheath does not burst, the arc-extinguishing material of the fuse tube between the end of the sheath and the exhaust end of the fuse tube is available for evolving gas in addition to that evolved from the sheath. The joint action of the two quantities of evolved gas, together with arc elongation, extinguish the arc.
Many manufacturers of cutouts and fuse links of the types described above continue to make concerted efforts to decrease the costs of both the material and labor thereof, both as a matter of simple, good commercial practice, and in view of the fact that certain materials, such as silver, copper and bronze, continue to experience large price increases. At the same time, manufacturers of cutouts and fuse links continue ongoing programs to improve the performance of these products. Evidence of one such attempt at improving fuse links is commonly assigned U.S. Pat. No. 4,272,572 issued June 9, 1981 to Bruce A. Biller and Hiram Jackson.
This last-named patent has as a goal the modification of the first terminal of the fuse link. Specifically, in typical prior art fuse links, the first terminal is an elongated metallic sleeve which is crimped around the flexible stranded cable and the ends of the fusible element and the strain wire. Elimination of the sleeve, it was surmised, would decrease the cost of the fuse link by eliminating material therefrom. However, some good and sure method of attaching the fuse link and the strain wire to the flexible cable had to be found. A solution to that problem is the subject matter of that patent. Briefly, it was found that compressing (by swaging of the like) a small length of the flexible, stranded cable into a somewhat semi-solid mass having a pocket formed therein, followed by placement of the ends of the fusible elements and the strain wire into the pocket, further followed by closure of the pocket by further compressing the cable, resulted in good mechanical and electrical connection between the flexible stranded cable and both the fusible element and the strain wire. Here, the compressed portion of the cable acts as, or in place of, the first terminal.
As an alternative to the invention of the last-named patent, a mere decrease in the length of the sleeve forming the lower terminal has been attempted. This clearly effects a material savings in fuse links. However, it was found that when the first terminal of the fuse link was either eliminated or shortened, certain problems in the operation of the fuse links could result. A first problem was that, due to the flexibility of the stranded cable, the shorter first terminal could become cocked within the sheath from which the terminal is very closely spaced. Cocking of the terminal, it was found, could result in wedging the edges of the terminal against the side walls of the sheath, thus inhibiting or preventing full movement of the first terminal and the cable out of the sheath and the fuse tube. From the above description of the fuse link and the fuse tube, it can be seen that if the first terminal and cable cannot fully and freely move out of the sheath and the fuse tube, arc interruption may not occur since insufficient arc elongation may result. A second problem involves the fact that forces on the first terminal, due to the gases evolved when the arc interacts with the arc-extinguishing material, push piston-like upon the first terminal. At times, this pushing force can cause the cable immediately adjacent the shorter first terminal to "baloon" or "mushroom" out. Such "ballooning" or "mushrooming" is manifested by a separation among, and a spreading out of, the individual strands of the flexible stranded cable. The ballooned or mushroomed portion of the cable could frictionally engage the side walls of the sheath, again inhibiting or preventing full movement of the first terminal and the cable out of the sheath and, ultimately, out of the fuse tube. This phenomenon had a deleterious effect upon the operation of the fuse link and the cutout.
The present invention is intended to solve both of the above described two problems, while achieving the overall goals of improving fuse link and cutout performance, and at the same time decreasing costs.